U.S. patent number 7,270,851 [Application Number 10/985,081] was granted by the patent office on 2007-09-18 for method for nanoencapsulation of aerogels and nanoencapsulated aerogels produced by such method.
This patent grant is currently assigned to N/A, The United States of America as represented by the Administrator of the National Aeronautics and Space Administration. Invention is credited to Thomas A. Sullivan.
United States Patent |
7,270,851 |
Sullivan |
September 18, 2007 |
Method for nanoencapsulation of aerogels and nanoencapsulated
aerogels produced by such method
Abstract
A method for increasing the compressive modulus of aerogels
comprising: providing aerogel substrate comprising a bubble matrix
in a chamber; providing monomer to the chamber, the monomer
comprising vapor phase monomer which polymerizes substantially free
of polymerization byproducts; depositing monomer from the vapor
phase onto the surface of the aerogel substrate under deposition
conditions effective to produce a vapor pressure sufficient to
cause the vapor phase monomer to penetrate into the bubble matrix
and deposit onto the surface of the aerogel substrate, producing a
substantially uniform monomer film; and, polymerizing the
substantially uniform monomer film under polymerization conditions
effective to produce polymer coated aerogel comprising a
substantially uniform polymer coating substantially free of
polymerization byproducts. Polymer coated aerogel comprising
aerogel substrate comprising a substantially uniform polymer
coating, said polymer coated aerogel comprising porosity and having
a compressive modulus greater than the compressive modulus of the
aerogel substrate, as measured by a 100 lb. load cell at 1
mm/minute in the linear range of 20% to 40% compression.
Inventors: |
Sullivan; Thomas A. (Houston,
TX) |
Assignee: |
The United States of America as
represented by the Administrator of the National Aeronautics and
Space Administration (Washington, DC)
N/A (N/A)
|
Family
ID: |
36262314 |
Appl.
No.: |
10/985,081 |
Filed: |
November 4, 2004 |
Prior Publication Data
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|
|
Document
Identifier |
Publication Date |
|
US 20060093808 A1 |
May 4, 2006 |
|
Current U.S.
Class: |
427/255.6;
427/249.2 |
Current CPC
Class: |
C01B
33/157 (20130101); C01B 33/16 (20130101); C09C
1/3072 (20130101); Y10T 428/24999 (20150401); Y10T
428/249991 (20150401); Y10T 428/249986 (20150401); Y10T
428/249992 (20150401); Y10T 428/249955 (20150401); C01P
2004/30 (20130101) |
Current International
Class: |
C23C
16/00 (20060101) |
Field of
Search: |
;427/255.6 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Larry L. Hench and Jon K. West, The Sol-Gel Process, Chemical
Reviews,1990, vol. 90 #1, pp. 33-72. cited by other .
J.K. Floess, R. Field, S. Rouanet, The use of vinyl functional
aerogels for reinforcement of silicone rubbers, Journal of
Non-Crystalline Solids 285, 2001, pp. 101-108. cited by other .
H. D. Gesser and P. C. Goswami, Aerogels and Related Porous
Materials, Chemical Reviews, 1989, vol. 89 #4, pp. 765-788. cited
by other .
Ulrich Schubert, Nicola Husing, and Anne Lorenz, Hybrid
Inorganic-Organic Materials by Sol-Gel Processing of
Organofunctional Metals Alkoxides, Chem. Mater., 1995, vol. 7, #11,
pp. 2010-2027. cited by other .
Jianye Wen and Garth L. Wilkes, Organic/Inorganic Hybrid Network
Materials by the Sol-Gel Approach, Chem. Mater. 1996, vol. 8, #8,
pp. 1667-1681. cited by other.
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Primary Examiner: Meeks; Timothy
Assistant Examiner: Stouffer; Kelly M.
Attorney, Agent or Firm: Ro; Theodore U.
Government Interests
The invention described herein was made by an employee of the
United States Government and may be manufactured and used by or for
the Government of the United States of America for governmental
purposes without the payment of any royalties thereon or therefor.
Claims
The invention claimed is:
1. A method for increasing the compressive modulus of aerogels
comprising: providing aerogel substrate comprising a bubble matrix
in a chamber; providing monomer to the chamber comprising vapor
phase monomer by vaporizing said monomer at a vaporization
temperature of about 200.degree. C. or less at about 1 atm or less
to produce said vapor phase monomer which polymerizes substantially
free of polymerization byproducts; depositing said monomer from the
vapor phase onto the surface of the aerogel substrate under
deposition conditions effective to produce a vapor pressure
sufficient to cause the vapor phase monomer to penetrate into the
bubble matrix and deposit onto the surface of the aerogel
substrate, producing a substantially uniform monomer film; and,
polymerizing the substantially uniform monomer film under
polymerization conditions effective to produce polymer coated
aerogel comprising a substantially uniform polymer coating
substantially free of polymerization byproducts, wherein said
depositing monomer comprises adsorbing monomer from the vapor phase
onto the surface of the aerogel substrate, and wherein said
polymerization conditions comprise an initiator; a polymerization
pressure of 1 atmosphere or more; a polymerization time of about 24
hours or less; and, a polymerization temperature of about
250.degree. C. or less.
2. The method of claim 1 wherein said monomer is selected from the
group consisting of monomer which hydrogen bonds with functional
groups at the aerogel surface and monomer which reacts with
functional groups at the aerogel surface.
3. The method of claim 2 wherein said monomer reacts with hydroxyl
groups at the aerogel surface.
4. The method of claim 3 wherein said monomer is selected from the
group consisting of monomers which polymerize by addition
reactions, difunctional cyclic monomers comprising complimentary
polymerizable groups, a combination of phenolic monomer and
formaldehyde, and 4-amino methylbenzoate.
5. The method of claim 4 further comprising a procedure selected
from the group consisting of evacuating the chamber and replacing
air in the chamber with inert gas.
6. The method of claim 5 further comprising providing monomer
precursor material in the chamber with the aerogel substrate;
sealing the chamber; and, introducing sufficient energy into the
system to produce said vapor phase monomer.
7. The method of claim 6 wherein said introducing sufficient energy
comprises increasing temperature in the chamber, said deposition
conditions further comprising reducing pressure in the chamber.
8. The method of claim 3 wherein said monomer is caprolactam.
9. The method of claim 3 wherein said monomer is caprolactone.
10. The method of claim 3 wherein said monomer is hexamethyl
cyclotrisiloxane.
11. The method of claim 2 wherein said vaporization temperature is
about 150.degree. C. or less.
12. The method of claim 2 wherein the monomer is olefin selected
from the group consisting of ethylene, propylene, butenes, dienes,
and combinations thereof.
13. The method of claim 2 wherein the monomer is selected from the
group consisting of isoprene, 1,3-butadiene, butene, isobutene, and
combinations thereof.
14. The method of claim 1 wherein said monomer is selected from the
group consisting of monomers which polymerize by addition
reactions, difunctional cyclic monomers comprising complimentary
polymerizable groups, a combination of phenolic monomer and
formaldehyde, and 4-amino methylbenzoate.
15. The method of claim 1 wherein the monomer is olefin selected
from the group consisting of ethylene, propylene, butenes, dienes,
and combinations thereof.
16. The method of claim 1 wherein the monomer is selected from the
group consisting of isoprene, 1,3-butadiene, butene, isobutene, and
combinations thereof.
17. The method of claim 1 wherein said monomer is caprolactam.
18. The method of claim 1 wherein said monomer is caprolactone.
19. The method of claim 1 wherein said monomer is hexamethyl
cyclotrisiloxane.
20. The method of claim 1 wherein said monomer is selected from the
group consisting of monomers which polymerize by addition
reactions, difunctional cyclic monomers comprising complimentary
polymerizable groups, a combination of phenolic monomer and
formaldehyde, and 4-amino methylbenzoate.
21. The method of claim 1 further comprising providing monomer
precursor material in the chamber with the aerogel substrate;
sealing the chamber; and, introducing sufficient energy into the
system to produce said vapor phase monomer.
22. The method of claim 1 wherein the substantially uniform polymer
coating increases the weight of the aerogel substrate by about 4
wt. % or more.
23. The method of claim 1 wherein the substantially uniform polymer
coating increases the weight of the aerogel substrate by about 4
wt. % or more.
24. The method of claim 1 wherein the aerogel substrate has a first
compressive modulus as measured by a 100 lb. load cell at 1
mm/minute in the linear range of 20% to 40% compression, and the
polymer coated aerogel has a compressive modulus which is about 20%
or more greater than the first compressive modulus.
25. The method of claim 1 wherein said deposition conditions and
said polymerization conditions are effective to produce polymer
coated aerogel having an R value of about 15 per inch or more.
26. The method of claim 1 wherein the initiator is selected from
the group consisting of acidic gas and alkaline gas.
27. The method of claim 1 wherein the initiator is selected from
the group consisting of energetic initiating agents, borohalides,
hydrogen chloride, and trimethyl amine.
28. The method of claim 1 further comprising treating the
polymer-coated aerogel with one or more post-polymerization
additives selected from the group consisting of waterproofing
agents, IR opacifiers, metal-binding moieties, selective binding
agents for chemical detection, stabilizers, microbial growth
inhibitors, and combinations thereof.
29. A method for increasing the compressive modulus of aerogels
comprising: providing aerogel substrate comprising a bubble matrix
in a chamber; providing monomer to the chamber comprising vapor
phase monomer which polymerizes substantially free of
polymerization byproducts; depositing said monomer from the vapor
phase onto the surface of the aerogel substrate under deposition
conditions effective to produce a vapor pressure sufficient to
cause the vapor phase monomer to penetrate into the bubble matrix
and deposit onto the surface of the aerogel substrate, producing a
substantially uniform monomer film; and, polymerizing the
substantially uniform monomer film under polymerization conditions
effective to produce polymer coated aerogel comprising a
substantially uniform polymer coating substantially free of
polymerization byproducts, wherein said monomer is selected from
the group consisting of monomer which hydrogen bonds with
functional groups at the aerogel surface and monomer which reacts
with functional groups at the aerogel surface.
30. The method of claim 29 wherein the substantially uniform
polymer coating increases the weight of the aerogel substrate by
about 4 wt. % or more.
31. The method of claim 29 wherein the aerogel substrate has a
first compressive modulus as measured by a 100 lb. load cell at 1
mm/minute in the linear range of 20% to 40% compression, and the
polymer coated aerogel has a compressive modulus which is about 20%
or more greater than the first compressive modulus.
32. The method of claim 29 wherein said deposition conditions and
polymerization conditions produce polymer coated aerogel having
effective porosity to retard heat transfer.
33. The method of claim 32 further comprising producing said
effective porosity by controlling monomer type, controlling monomer
loading, and combinations thereof.
34. The method of claim 29 further comprising coating the aerogel
substrate with an insulating coating effective to suppress
infra-red heat transfer.
35. The method of claim 34 wherein said insulating coating
comprises an element selected from the group consisting of carbon
and metal oxide.
36. The method of claim 34 wherein said insulating coating is
selected from the group consisting of titanium dioxide and
carbon.
37. The method of claim 34 further comprising feeding a gaseous
additive precursor to the chamber; and, subjecting the gaseous
additive precursor to chemical vapor deposition conditions
effective to decompose said gaseous additive precursor and to
produce said insulating coating.
38. The method of claim 37 wherein the gaseous additive precursor
is selected from the group consisting of gaseous hydrocarbons and
titanium-containing gases.
39. The method of claim 37 wherein said gaseous additive precursor
is selected from the group consisting of methane and titanium
chloride.
40. The method of claim 29 wherein said deposition conditions and
said polymerization conditions are effective to produce polymer
coated aerogel having an R value of about 15 per inch or more.
41. The method of claim 29 wherein the polymer coated aerogel
comprises a polymer content of from about 10% to about 95%.
42. The method of claim 29 wherein said deposition conditions and
said polymerization conditions are effective to produce polymer
coated aerogel having an effective Sound Transmission Class (STC)
rating for a given application.
43. The method of claim 29 further comprising treating the aerogel
substrate to produce a surface comprising compatibilizing
groups.
44. The method of claim 43 further comprising one or more features
selected from the group consisting of providing monomers and
compatibilizing groups both comprising one or more polymerizable
unsaturated carbon-carbon bond; providing monomer comprising one or
more amide bond and compatibilizing groups comprising organic amino
groups effective to produce organically modified silica effective
to form amide bonds with the polymer coating; and, providing
compatibilizing group(s) effective to render the surface of the
aerogel substrate more organic.
45. The method of claim 43 further comprising providing monomers
comprising one or more polymerizable unsaturated carbon-carbon
bond; and, providing compatilibizing groups selected from the group
consisting of allylalkylchlorosilanes, vinylalkylchlorosilanes, and
congeners thereof comprising unsaturated carbon-carbon bond(s) and
having up to about 20 carbon atoms.
46. The method of claim 43 further comprising providing monomers
comprising one or more polymerizable unsaturated carbon-carbon
bond; and, providing compatibilizing groups selected from the group
consisting of vinyldimethylchlorosilane, allyl dimethyl
chlorosilane, and combinations thereof.
47. The method of claim 43 comprising providing monomer comprising
one or more amide bond; and, providing compatibilizing groups
selected from the group consisting of 3-aminopropyl
trimethoxysilane and aminophenyl trimethyoxysilane (APTMOS).
48. The method of claim 29 further comprising silylating the
surface of the aerogel substrate to incorporate silylating agent
comprising compatibilizing groups.
49. The method of claim 48 wherein the silylating agent is
effective to react with surface -OH groups and to form covalent
Si-O-Si bonds comprising one or more compatibilizing groups which
copolymerize with polymerizable groups in the monomer film.
50. The method of claim 48 wherein said silylating agent is
selected from the group consisting of organo-chlorosilanes and
disiloxanes.
51. The method of claim 29 further comprising pumping remaining
unreacted monomer out of the chamber.
52. A method for increasing the compressive modulus of aerogels
comprising: providing aerogel substrate comprising a bubble matrix
in a chamber; providing monomer to the chamber comprising vapor
phase monomer which polymerizes substantially free of
polymerization byproducts; depositing said monomer from the vapor
phase onto the surface of the aerogel substrate under deposition
conditions effective to produce a vapor pressure sufficient to
cause the vapor phase monomer to penetrate into the bubble matrix
and deposit onto the surface of the aerogel substrate, producing a
substantially uniform monomer film; and, polymerizing the
substantially uniform monomer film under polymerization conditions
effective to produce polymer coated aerogel comprising a
substantially uniform polymer coating substantially free of
polymerization byproducts, wherein the monomer comprises
difunctional cyclic monomer comprising complimentary polymerizable
groups selected from the group consisting of acyl groups, amine
groups, thiol groups, hydroxyl groups, cyanate groups, groups
comprising polymerizable unsaturated carbon-carbon bonds, oxirane
groups, amide groups, ester groups, cyclic siloxanes, pyrroles,
organosilanes, and combinations thereof.
53. The method of claim 52 wherein the substantially uniform
polymer coating increases the weight of the aerogel substrate by
about 4 wt. % or more.
54. The method of claim 52 wherein the aerogel substrate has a
first compressive modulus as measured by a 100 lb. load cell at 1
mm/minute in the linear range of 20% to 40% compression, and the
polymer coated aerogel has a compressive modulus which is about 20%
or more greater than the first compressive modulus.
55. The method of claim 52 wherein said deposition conditions and
said polymerization conditions are effective to produce polymer
coated aerogel having an R value of about 15 per inch or more.
56. The method of claim 52 wherein the polymer coated aerogel
comprises a polymer content of from about 10% to about 95%.
57. A method for increasing the compressive modulus of aerogels
comprising: providing aerogel substrate comprising a bubble matrix
in a chamber; providing monomer to the chamber comprising vapor
phase monomer which polymerizes substantially free of
polymerization byproducts; depositing said monomer from the vapor
phase onto the surface of the aerogel substrate under deposition
conditions effective to produce a vapor pressure sufficient to
cause the vapor phase monomer to penetrate into the bubble matrix
and deposit onto the surface of the aerogel substrate, producing a
substantially uniform monomer film; and, polymerizing the
substantially uniform monomer film under polymerization conditions
effective to produce polymer coated aerogel comprising a
substantially uniform polymer coating substantially free of
polymerization byproducts, wherein the monomer is selected from the
group consisting of olefins, vinyl carboxylates, vinyl ethers,
epoxides, acrylates, methacrylates, cyanoacrylates, formaldehyde,
lactams, lactones, hexaorganocyclotrisiloxanes,
dichlorodiorganosilanes, and phenols.
58. The method of claim 57 wherein the substantially uniform
polymer coating increases the weight of the aerogel substrate by
about 4 wt. % or more.
59. The method of claim 57 wherein the aerogel substrate has a
first compressive modulus as measured by a 100 lb. load cell at 1
mm/minute in the linear range of 20% to 40% compression, and the
polymer coated aerogel has a compressive modulus which is about 20%
or more greater than the first compressive modulus.
60. The method of claim 57 wherein said deposition conditions and
said polymerization conditions are effective to produce polymer
coated aerogel having an R value of about 15 per inch or more.
61. The method of claim 57 further comprising treating the
polymer-coated aerogel with one or more post-polymerization
additives selected from the group consisting of waterproofing
agents, IR opacifiers, metal-binding moieties, selective binding
agents for chemical detection, stabilizers, microbial growth
inhibitors and combinations thereof.
62. A method for increasing the compressive modulus of aerogels
comprising: providing aerogel substrate comprising a bubble matrix
in a chamber; providing monomer to the chamber comprising vapor
phase monomer which polymerizes substantially free of
polymerization byproducts; depositing said monomer from the vapor
phase onto the surface of the aerogel substrate under deposition
conditions effective to produce a vapor pressure sufficient to
cause the vapor phase monomer to penetrate into the bubble matrix
and deposit onto the surface of the aerogel substrate, producing a
substantially uniform monomer film; polymerizing the substantially
uniform monomer film under polymerization conditions effective to
produce polymer coated aerogel comprising a substantially uniform
polymer coating substantially free of polymerization byproducts,
wherein the substantially uniform polymer coating increases the
weight of the aerogel substrate by about 4 wt. % or more; and a
procedure selected from the group consisting of evacuating the
chamber and replacing air in the chamber with inert gas.
63. The method of claim 62 wherein the aerogel substrate has a
first compressive modulus as measured by a 100 lb. load cell at 1
mm/minute in the linear range of 20% to 40% compression, and the
polymer coated aerogel has a compressive modulus which is about 20%
or more greater than the first compressive modulus.
64. The method of claim 62 wherein said deposition conditions and
said polymerization conditions are effective to produce polymer
coated aerogel having an R value of about 15 per inch or more.
65. A method for increasing the compressive modulus of aerogels
comprising: providing to a chamber ethylcyanoacrylate in gel form
and aerogel substrate comprising a bubble matrix; exposing the
chamber to vaporization conditions comprising a vaporization
temperature, a vaporization pressure, and a vaporization time
effective to produce an ethylcyanoacrylate vapor pressure
sufficient to cause ethylcyanoacrylate monomer to vaporize and to
penetrate into the bubble matrix; exposing the chamber to
deposition conditions comprising a deposition temperature, a
deposition pressure, and a deposition time effective to cause
ethylcyanoacrylate monomer to adsorb onto the surface of the
aerogel substrate from vapor phase, producing a substantially
uniform ethylcyanoacrylate monomer film; and, polymerizing the
substantially uniform ethylcyanoacrylate monomer film under
polymerization conditions comprising a polymerization temperature,
a polymerization pressure, and a polymerization time effective to
produce polymer coated aerogel comprising a substantially uniform
polymer coating consisting essentially of
poly(ethylcyanoacrylate).
66. The method of claim 65 wherein the vaporization conditions, the
deposition conditions, and the polymerization conditions comprise a
total time of from about 45 hours or less.
67. The method of claim 65 wherein the vaporization conditions, the
deposition conditions, and the polymerization conditions comprise a
temperature of about 40 to about 50.degree. C. and a total time of
from about 70 hours or less.
68. A method for increasing the compressive modulus of aerogels
comprising: providing to a chamber caprolactam and aerogel
substrate comprising a bubble matrix; exposing the chamber to
vaporization conditions comprising a vaporization temperature, a
vaporization pressure, and a vaporization time effective to produce
a caprolactam vapor pressure sufficient to cause caprolactam
monomer to vaporize and to penetrate into the bubble matrix;
exposing the chamber to deposition conditions comprising a
deposition temperature, a deposition pressure, and a deposition
time effective to cause caprolactam monomer to adsorb onto the
surface of the aerogel substrate from vapor phase, producing a
substantially uniform caprolactam monomer film; and, polymerizing
the substantially uniform caprolactam monomer film under
polymerization conditions comprising a polymerization temperature,
a polymerization pressure, and a polymerization time effective to
produce polymer coated aerogel comprising a substantially uniform
polymer coating consisting essentially of nylon-6.
69. The method of claim 68 wherein the vaporization conditions, the
deposition conditions, and the polymerization conditions comprise
temperature of from about 60.degree. C. to about 65.degree. C. and
a total time of from about 2.5 days or more.
70. The method of claim 68 wherein the vaporization temperature and
the deposition temperature are about 150.degree. C. and the
combined vaporization time and deposition time is about 24 hours or
less; and the polymerization temperature is about 250.degree. C. or
less and the polymerization time is about 1 hour or less.
Description
FIELD OF THE APPLICATION
The application relates to a method for nanoencapsulating aerogels
and to nanoencapsulated aerogels produced by such method.
BACKGROUND OF THE APPLICATION
Aerogels are nanoporous, lightweight materials with very low
thermal and acoustic conductivity and high porosity. Not
surprisingly, aerogels tend to lack mechanical strength. Various
attempts have been made to increase the mechanical strength of
aerogels; however, such methods tend to adversely affect desired
aerogel properties.
BRIEF SUMMARY
The application provides a method for increasing the compressive
modulus of aerogels. The method comprises: providing aerogel
substrate comprising a bubble matrix in a chamber; providing
monomer to the chamber comprising vapor phase monomer which
polymerizes substantially free of polymerization byproducts;
depositing monomer from the vapor phase onto the surface of the
aerogel substrate under deposition conditions effective to produce
a vapor pressure sufficient to cause the vapor phase monomer to
penetrate into the bubble matrix and deposit onto the surface of
the aerogel substrate, producing a substantially uniform monomer
film; and, polymerizing the substantially uniform monomer film
under polymerization conditions effective to produce polymer coated
aerogel comprising a substantially uniform polymer coating
substantially free of polymerization byproducts.
The application also provides polymer coated aerogel comprising
aerogel substrate comprising a substantially uniform polymer
coating, said polymer coated aerogel comprising porosity and having
a compressive modulus greater than the compressive modulus of the
aerogel substrate, as measured by a 100 lb. load cell at 1
mm/minute in the linear range of 20% to 40% compression.
DETAILED DESCRIPTION
Aerogels generally are produced by drying a suitable gel. The term
"gel" refers to a coherent, rigid, continuous three-dimensional
network of colloidal particles and the residual solution in which
the network was formed. Gels are produced by the aggregation of
colloidal particles, typically under acidic conditions, to form a
three dimensional gel microstructure. Aerogel forms when the gel is
dried, or liquid is removed from the pores of the gel, leaving the
gel microstructure preserved.
Aerogels generally are inorganic, organic, or combinations thereof.
Organic aerogels are derived from widely diverse starting
materials, e.g. melamine formaldehyde. Inorganic aerogels generally
comprise a colloidal component. Suitable colloidal components
comprise an element including, but not necessarily limited to
elements selected from the group consisting of silicon, aluminum,
titanium, zirconium, tin, cerium, vanadium, hafnium, and mixtures
thereof. Methods for producing inorganic aerogels are described in
H. D. Gesser, P. C. Goswarni, Chem Rev. 1989, 89, 765 et seq.,
incorporated herein by reference. The preferred colloidal component
will differ depending upon the application. For most purposes, a
preferred colloidal component is silicon dioxide or silica.
Aerogels typically possess a nanostructure that is composed of
extremely small interconnected particles with diameters of from 2
to about 10 nanometers and which surround pore spaces that are
typically less than 100 nm and have an average or mean pore
diameter of about 20 nm to about 100 nm. The interconnected
particles produce a solid material comprising pores, sometimes
called a "bubble matrix," with ultrafine pore size, low density,
high surface area (350-1500 m.sup.2/g), and open-cell porosity. The
nanostructure of aerogels give them unusual acoustic, optical,
thermal, and mechanical properties. As a result of their particle
size, aerogels do not appreciably scatter visible light and can be
made transparent. The high porosity and small pore size of aerogels
also makes aerogels lightweight, super-insulating materials.
Common physical properties of silica aerogels are listed in Table 1
below:
TABLE-US-00001 TABLE 1 PROPERTY VALUE Density 0.003-0.35 g/cm.sup.3
Internal Surface Area 350-1500 m.sup.2/g Mean Pore Diameter 20-100
nm Primary Particle Diameter 2-10 nm
Internal surface area is calculated based on the amount of nitrogen
adsorbed at five different relative pressures over the range 0.05
to 0.25 according to the Brunauer-Emmett-Teller (BET) model, which
is referenced in Gregg, S. J. and Sing, K. S. W, "Adsorption,
Surface Area and Porosity," p. 285, Academic Press, New York
(1991), incorporated herein by reference.
The foregoing properties may be significantly affected by the
conditions used to prepare the aerogels and any subsequent
post-processing.
In a preferred embodiment, the bubble matrix may be empty (vacuum),
or may contain one or more gas(es). In a preferred embodiment, the
bubble matrix contains air; however, the bubble matrix may contain
substantially any environmentally compatible gas, including but not
necessarily limited to gases comprising oxygen, helium, nitrogen,
carbon dioxide, and other inert elements, or other low activity
gases.
Method for Strengthening Aerogels
As explained above, aerogels tend to be relatively fragile. The
application provides a method for increasing the mechanical
strength of aerogels. A substantially uniform monomer film is
deposited on the aerogel substrate and polymerized.
As a consequence of strengthening the aerogel, the polymer coating
improves the integrity of the treated material. Untreated aerogel
is subject to the formation of small fragments that break off the
main body of the material during handling, flexing, compression,
etc. When the aerogel substrate is encapsulated in an adequate
layer of polymer, however, the material is less subject to such
particulate formation. This is especially true when the polymer is
bound to the aerogel substrate by the incorporation of a
compatibilizing group on the surface of the aerogel (discussed more
fully below).
Suitable Monomers
Substantially any vaporizable or sublimable polymerizable monomers
may be used in the process as long as the monomers vaporize and
deposit onto the aerogel within a commercially reasonable period of
time at a pressure of 1 atmosphere or below at temperatures
sufficiently low to avoid damaging the monomer or the aerogel
substrate. A commercially reasonable period of time will vary
depending upon the value of the polymer coated aerogel. For most
monomers and aerogel substrates, a commercially reasonable period
of time is 24 hours or less. At temperatures of about 200.degree.
C. or less, preferably about 150.degree. C. or less, at pressures
of 1 atm or lower, preferred monomers generate sufficient vapor
pressure to cause the vapor phase monomer to penetrate into the
bubble matrix and deposit onto the surface of the aerogel
substrate, producing a "substantially uniform monomer film"
(defined below). Preferred monomers also tend to have an inherent
attraction to the surface of the aerogel substrate under
vapor-phase conditions. The type of inherent attraction may vary
depending on the aerogel substrate and its surface properties. In a
preferred embodiment, the monomers tend to form hydrogen bonds with
the surface of the aerogel substrate and/or to react with hydroxyl
groups at the surface of the aerogel substrate.
Suitable monomers produce polymer coating having desired
properties, including but not necessarily limited to strength,
flexibility, non-flammability, elasticity, thermal stability,
electrical conductivity, and transparency. Suitable properties also
may involve, but are not necessarily limited to stability to
ultraviolet light, ability to absorb infrared or ultraviolet light,
and/or stability to radiation. Suitable polymer coatings include,
but are not necessarily limited to polyamides, polyesters,
polysiloxanes, polyacetals, polyaramides, polyolefins,
polypyrroles, parylene polymers, thermosetting resins, and
polyacrylates. Suitable polyacrylates include, but are not
necessarily limited to polyacrylates, polymethacrylates, and
polycyanoacrylates. Preferred thermosetting resins are produced
from the reaction of phenolic compound and formaldehyde. Preferred
polymer coatings include, but are not necessarily limited to
polyamides and polyolefins. Preferred polyamides include, but are
not necessarily limited to nylon, preferably nylon-6. Preferred
polyolefins include but are not necessarily limited to
polyacrylates.
Suitable monomers deposit, preferably by adsorption, onto the
surface of individual filaments of the aerogel substrate under
vapor phase conditions. The propensity of monomers to deposit onto
the surface of the aerogel substrate depends on the polarity of the
monomers and the nature of the surface of the aerogel substrate
(i.e., the number of hydrogen-bonding sites). Where the surface of
the aerogel substrate has a high oxygen content, as with silica
aerogels, monomers with more polar groups tend to adsorb onto the
surface of the monomer more readily.
Suitable monomers polymerize by a reaction which produces polymer
substantially free of byproducts. Suitable monomers polymerize by
any reaction type, including but not necessarily by addition and
condensation reactions. Preferred monomers are selected from the
group consisting of monomers which polymerize by addition
reactions, difunctional cyclic monomers comprising complimentary
polymerizable groups, a combination of phenolic monomer and
formaldehyde, and 4-amino methylbenzoate.
Addition reactions include, but are not necessarily limited to free
radical addition reactions, anionic addition reactions, and
cationic addition reactions. Examples of monomers that undergo
addition reactions include, but are not necessarily limited to
monomers comprising polymerizable unsaturated carbon-carbon
bonds.
Examples, of polymers produced by condensation reactions include,
but are not necessarily limited to polyamides and polyesters.
Monomers which polymerize via condensation reactions generally
produce by-products, such as water, alcohol, HCl, combinations
thereof, depending on the particular starting materials. The
production of byproducts can be avoided in condensation reactions,
and in other types of reactions, using difunctional cyclic monomers
comprising complimentary polymerizable groups. In a preferred
embodiment, the complimentary polymerizable groups comprise an
element selected from the group consisting of oxygen and nitrogen.
Examples of preferred difunctional cyclic monomers comprising
complimentary polymerizable groups include, but are not necessarily
limited to caprolactam (which produces nylon-6), caprolactone
(which produces polyester), and hexamethylcyclotrisiloxane (which
produces poly-dimethylsiloxane).
A most preferred difunctional cyclic monomer is caprolactam. Both
stoichiometry and by-product concerns are eliminated using
caprolactam. Caprolactam is believed to have a strong affinity for
the aerogel surface due to hydrogen bonding and reactivity with
silanol groups on the aerogel to provide a covalent link to the
silica network. The nylon-6 polymer coating produced using
caprolactam also has many opportunities to hydrogen-bond with the
aerogel substrate.
Another suitable monomer is 4-amino methyl benzoate, which
polymerizes to provide a polyaramid (poly-p-benzamide) that can be
expected to have high strength and low flammability. This
embodiment suffers from the disadvantage, however, that by-product
methanol requires removal from the system.
Substantially any complimentary polymerizable groups may be used.
Suitable polymerizable groups include, but are not necessarily
limited to those selected from the group consisting of acyl groups,
amine groups, thiol groups, hydroxyl groups, cyanate groups, groups
comprising polymerizable unsaturated carbon-carbon bonds, oxirane
groups, amide groups, ester groups, cyclic siloxanes, pyrroles,
organosilanes, paracyclophanes, and combinations thereof.
Examples of the foregoing monomers include, but are not necessarily
limited to olefins, vinyl carboxylates, vinyl ethers, epoxides,
acrylates, methacrylates, cyanoacrylates, formaldehyde, lactams,
lactones, hexaorganocyclotrisiloxanes, dichlorodiorganosilanes,
[2.2] paracyclophane, and phenols. Examples of suitable olefins
include, but are not necessarily limited to ethylene, propylene,
butene, and dienes, including but not necessarily limited to
isoprene and 1,3-butadiene, optionally with butene or
isobutene.
Formation of Monomer Film
In order to deposit the monomer, the aerogel substrate is placed in
a chamber, preferably a sealed chamber. The sealed chamber may or
may not be evaculated. Air may be present in the chamber, or it may
be replaced with an inert gas to reduce flammability potential, to
prevent premature polymerization, and/or to increase the rate of
diffusion of the monomer into the aerogel network.
Monomer is provided to the chamber either before or after the
substrate is placed in the chamber. The monomer may be a solid, a
liquid, or a gas, depending on the properties of the monomer,
taking into consideration any process and safety concerns.
In a preferred embodiment, monomer precursor material is placed in
the chamber with the aerogel substrate and the chamber is sealed.
Monomer vapor typically evolves upon the introduction of energy
into the system. In a preferred embodiment, the temperature is
controlled and the energy comprises an increase in temperature. The
pressure and/or temperature is adjusted causing a vapor pressure
sufficient to cause the monomer to penetrate into the bubble matrix
and deposit onto the surface of the aerogel substrate to produce a
substantially uniform monomer film. Although not required, a
vacuum, or reduced pressure, is helpful to remove pore-filling
gasses and to increase the rate of permeation and deposition of the
monomer into the network of the aerogel substrate. The monomer
vapor also may be generated outside of the chamber and introduced
into the chamber as a vapor. The deposition temperature and
pressure will vary depending upon the monomer.
Deposition is continued until a "substantially uniform" monomer
film is formed on the surface of the aerogel substrate. As used
herein, the phrase "substantially uniform" means a monomer film
which produces a polymer coating upon polymerization which
maintains effective porosity while increasing the compressive
modulus of the aerogel, as measured by a 100 lb. load cell at 1
mm/minute in the linear range of 20% to 40% compression.
Preferably, the monomer thickness is effective to produce a polymer
coated aerogel having a compressive modulus which is about 20% or
more greater, more preferably about 25% or more greater, even more
preferably by about 40% or more greater than the compressive
modulus of the aerogel substrate. Generally, the monomer film
comprises a quantity of monomer effective to increase the weight of
the resulting polymer coated aerogel by about 4 wt. % or more
compared to the aerogel substrate, preferably by about 10 wt. % or
more, more preferably by about 50 wt. % or more.
A "substantially uniform" monomer film or polymer coating does not
mean 100% of the internal surface area of the aerogel substrate is
coated with the monomer film, or that the thickness of the monomer
film (or the resulting polymer coating) necessarily is the same
over the 100% of the internal surface area of the aerogel
substrate. In fact, due to the physics of liquid films adhering to
solids, a "substantially uniform" monomer film and polymer coating
may have different thicknesses at different locations within the
aerogel network. Points of contact between the particles which make
up the network, known as "necks," are likely to have a thicker
monomer film and thus a thicker polymer coating. The thicker
polymer coating at the necks tends to strengthen the necks, which
otherwise tend to be relatively weak in terms of modulus.
The strength and insulating power of the polymer coated aerogel is
varied to meet the needs of a particular product by varying the
density and porosity of the starting aerogel substrate and/or by
varying the type of monomer used to form the polymer coating, the
loading of monomer, and thus the "effective porosity" of the
resulting polymer coated aerogel.
Where the polymer coated aerogel is for use as thermal insulation
in a rigid item, such as a refrigerator, "effective porosity" is a
porosity effective to retard heat transfer. Denser aerogels have a
slightly higher thermal conductivity than less dense aerogels.
Polymer coated aerogels are denser, and therefore have a slightly
higher thermal conductivity than aerogel substrate, but are still
effective insulators. The density of a polymer coated aerogel
depends upon the density of the aerogel substrate and the amount
and type of polymer coating on the aerogel substrate. The slightly
higher thermal conductivity of polymer coated aerogel can be
decreased by coating the aerogel substrate with an insulating
coating effective to suppress infra-red heat transfer. Suitable
insulating coatings comprise elements including, but are not
necessarily limited to carbon, titania, and combinations
thereof.
The common measure of insulating value is calculated as R, which is
the resistance to heat flow. Typical residential fiberglass batting
in the 3.5 inch gap of a wall has an R value of from about 10 to
about 13, or from about 3 to about 4 per inch. One inch of aerogel
provides thermal insulation equivalent to about 10 inches of
fiberglass. Based on the values in the following Table, an
effective porosity for a polymer coated aerogel for use as thermal
insulation produces an R value per inch of 15 or more, preferably
20 or more, more preferably about 25 or more, most preferably about
30 to 40 per inch. The R value of polymer coated aerogels is very
high, which makes them extremely efficient as thermal
insulation:
TABLE-US-00002 Insulation R-Value Material R value (ft{circumflex
over ( )}2.degree. F. h/Btu Hardwood siding (1 in. thick) 0.91
Brick (4 in. thick) 4.00 Fiberglass batting (3.5 in. thick) 10.90
Fiberglass batting(6 in. thick) 18.80 Fiberglass board (1 in.
thick) 4.35 Cellulose fiber (1 in. thick) 3.70 Flat glass (0.125 in
thick) 0.89 Insulating glass (0.25 in space) 1.54 Air space (3.5
in. thick) 1.01 Free stagnant air layer 0.17 Sheathing (0.5 in.
thick) 1.32
Depending on the particular use of the insulating material, the use
of polymer coated aerogel can enable greater energy efficiency,
volume efficiency, or even enable a product where none could be
developed at all. For instance, appliances such as ovens and
refrigerators can incorporate polymer coated aerogel insulation and
dramatically reduce power requirements.
As the need for strength or flexibility increases, the process is
controlled to provide for greater monomer loading. The polymer
content of the polymer coated aerogel generally ranges from 10% to
95%, more preferably 20% to 90%.
Where the polymer coated aerogel is for use as an acoustic
insulation material, an effective porosity is a porosity effective
to produce the Sound Transmission Class (STC) rating required for a
given application.
Modification of the Surface of the Aerogel Substrate
Factors that influence adsorption of monomer onto the surface of
the aerogel include, but are not necessarily limited to the surface
tension of the monomer film, the surface energy of the aerogel, and
the binding energy of the monomer. Monomer adsorption can be
improved by treating the aerogel substrate to incorporate
"compatibilizing groups," or groups which produce a treated aerogel
substrate surface having a greater affinity for given monomers
and/or groups which copolymerize with given monomers. Treating the
aerogel substrate can be accomplished either during manufacture of
the aerogel substrate or after manufacture of the aerogel
substrate.
For exemplary methods for treating the aerogel substrate during
manufacture, see the following references, each of which is
incorporated herein by reference: The use of vinyl functional
aerogels for reinforcement of silicone rubbers, J. K. Floess, R.
Field, S. Rouanet, J. Non-Cryst. Sol., 285, 101-108 (2001);
Organic/Inorganic Hybrid Network Materials by the Sol-Gel Approach,
J. Wen and G. L. Wilkes, Chem. Mater., 8, 1996 p1667-1681; Hybrid
Inorganic-Organic Materials by Sol-Gel Processing of
Organofunctional Metal Alkoxides, U. Schubert, N. Husing, and A.
Lorenz, Chem.
Mater. 1995, 7, 2010-2027; and, The Sol-Gel Process, L. L. Hench
and J. K. West, Chem. Rev., 1990, 90, 33-72.
In a preferred embodiment, the aerogel surface is silylated to
incorporate silylating agents comprising compatibilizing groups. In
one embodiment, the surface of the aerogel substrate is pretreated
with silylating agent effective to react with surface --OH groups
and to form covalent Si--O--Si bonds comprising one or more
compatibilizing groups which copolymerize with polymerizable groups
in the monomer film.
Where the monomers comprise polymerizable unsaturated carbon-carbon
bond(s), the compatibilizing groups preferably comprise one or more
polymerizable unsaturated carbon-carbon bond(s). Examples of
compatibilizing groups for use with monomers comprising unsaturated
carbon-carbon bonds include, but are not necessarily limited to
allylalkylchlorosilanes, vinylalkylchlorosilanes, and congeners
thereof comprising unsaturated carbon-carbon bond(s) having up to
about 20 carbon atoms. Preferred silylating agents comprising an
unsaturated polymerizable carbon-carbon bond include, but are not
necessarily limited to vinyldimethylchlorosilane, and allyl
dimethyl chlorosilane.
Where the monomer comprises an amide bond, preferred
compatibilizing groups comprise organic amino groups effective to
produce organically modified silica effective to form amide bonds
with the polymer coating. Examples of suitable silylating agents
comprising organic amino groups include, but are not necessarily
limited to 3-aminopropyl trimethoxysilane and aminophenyl
trimethyoxysilane (APTMOS).
In one embodiment, the silylating agent comprises compatibilizing
group(s) effective to render the surface of the aerogel substrate
more organic, thereby improving adsorption of the monomer film. In
this embodiment, suitable silylating agents include, but are not
necessarily limited to organo-chlorosilanes and disiloxanes.
Additives
It may be desirable to coat the aerogel substrate with one or more
additives, either before or after polymerization of the monomers.
Suitable additives include, but are not necessarily limited to
additives effective to opacify the polymer coated aerogel, to
decrease thermal conductivity, and/or to strengthen the polymer
coated aerogel. Suitable additives comprise an element including,
but not necessarily limited to carbon and metal (preferably metal
oxide). In a preferred embodiment, the additives are selected from
the group consisting of carbon and titanium dioxide.
Where the additive is deposited before the monomer film, the
additive is deposited onto the surface of the aerogel substrate
using any method effective to produce sufficient permeation into
the bubble matrix. In one embodiment, the additive is deposited
onto the aerogel substrate using chemical vapor deposition.
Suitable additives for chemical vapor deposition, or other
techniques, include, but are not necessarily limited to carbon and
titanium dioxide.
In order to deposit the additive using chemical vapor deposition,
an additive precursor is fed to the chamber. The additive precursor
preferably is gaseous under chemical vapor deposition conditions (a
"gaseous additive precursor"), and decomposes to produce a film of
the desired additive. Suitable additive precursors for carbon
include hydrocarbons, including but not necessarily limited to
gaseous hydrocarbons, preferably methane. Suitable additive
precursors for titanium dioxide include, but are not necessarily
limited to titanium-containing gases, preferably titanium chloride.
Titanium chloride hydrolyzes to form titanium dioxide in situ.
After the additive is deposited onto the aerogel substrate, the
monomer film is deposited onto the aerogel substrate and
polymerized to produce the polymer coating.
This embodiment is useful when it is desirable to protect an
additive during subsequent handling of the polymer coated aerogel.
For example, where the additive is carbon, deposition of the
additive prior to polymerization avoids the production of black
dust in later handling and use.
Polymerization of Monomer Film
Either substantially simultaneous with deposition or after
deposition of a substantially uniform monomer film, the monomer
film is subjected to polymerization conditions effective to
polymerize the monomers and to produce a substantially uniform
polymer coating. The polymerization conditions vary with the
monomer, but generally comprise a catalyst or initiator, a
controlled temperature, a pressure of about 1 atm or more, and a
time of about 24 hours or less. Preferably, the temperature is
about 250.degree. C. or less, more preferably about 200.degree. C.
or less.
The type of initiator will vary depending upon the type of monomer.
Certain monomers are activated by gaseous initiating agents,
including but not necessarily limited to acidic or alkaline gases.
Examples of such gases include, but are not necessarily limited to
borohalides, hydrogen chloride, trimethyl amine, and ammonia. A
preferred borohalide is BF.sub.3. Other monomers are activated by
energetic initiating agents. Examples of energetic initiating
agents include, but are not necessarily limited to light, heat,
electromagnetic energy, and electrical energy. Suitable electrical
energy includes, but is not necessarily limited to an electron
beam. For some monomers, it may be necessary to deposit initiator
onto the aerogel substrate.
Once a substantially uniform polymer coating is formed on the
aerogel substrate, remaining unreacted monomer preferably is pumped
out of the chamber. The formation of monomer film and polymer
coating may be simultaneous or stepwise, and may be repeated
several times (if necessary) until an effective polymer coating is
formed.
Post Polymerization Treatments
The polymer-coated aerogel also may be treated with post
polymerization additives, which may be volatile or non-volatile.
Suitable post polymerization additives include, but are not
necessarily limited to waterproofing agents, IR opacifiers,
metal-binding moieties, selective binding agents for chemical
detection, stabilizers, and microbial growth inhibitors. Preferred
post polymerization additives are waterproofing agents,
stabilizers, carbon, and titanium dioxide.
The method will be better understood with reference to the
following examples, which are illustrative only:
EXAMPLE 1
A sample of silica aerogel 2.times.3.times.4 cm (0.5532 g) was
placed into a lightly closed container with 0.864 g of commercially
available "super-glue" in gel form (.about.90% ethylcyanoacrylate).
The container was placed into an oven at 65-70.degree. C. for 47
hours. Upon removal from the oven, much of the monomer had hardened
on the bottom of the container, but some of monomer had
equilibrated into the aerogel network and then polymerized. The
treated sample weighed 0.7322 g, having gained 32% in weight. This
equates to a 24.4% cyanoacrylate coating in the product.
The sample was subjected to compression testing on a MTS Qtest/25
machine with a 100 lb load cell at 1 mm/minute in the linear range
of 20% to 40% compression. The modulus was determined to be 0.557
kg/cm.sup.2 (0.0546 MPa). Infra-red analysis of the product
confirmed the presence of poly(ethylcyanoacrylate) in the aerogel
network.
EXAMPLE 2
A sample of silica aerogel (0.6191 g) was similarly treated as in
Example 1, using 1.33 g of super-glue in gel form. The container
was placed into an oven at 48.degree. C. for 74 hours. The treated
sample weighed 0.8510 g, but still had a strong odor of
ethylcyanoacrylate. A few drops of water were added to the
container (not touching the aerogel) and the closed container sat
overnight at ambient temperature to ensure completion of
polymerization.
This sample was tested for compressive modulus using the same
instrument and method as in Example 1, resulting in a value of
0.556 kg/cm.sup.2 (0.0545 MPa.) Two control samples from the same
batch of aerogel provided modulus values of 0.392 kg/cm.sup.2
(0.0384 MPa) and 0.382 kg/cm.sub.2 (0.0375 MPa.) Thus, the treated
samples had a modulus roughly 44% higher than the untreated
samples.
EXAMPLE 3
A sample of silica aerogel was exposed to vapors of caprolactam by
heating the sample in a lightly closed container with the monomer
at 62.degree. C. for 2.5 days. The treated sample gained 23.7% in
mass, equating to 19.2% caprolactam in the sample. The modulus of
this sample was 0.487 Kg/cm.sup.2, representing an improvement of
approximately 25% over the untreated controls.
An IR spectrum of the material confirmed the deposition of organics
onto the aerogel, but the wavenumber of the carbonyl group was
shifted slightly higher than in bulk caprolactam. This peak was at
the same location (1640 cm.sup.-1) as the carbonyl peak for
nylon-6, but a second peak of nylon-6, which was located at 1540
cm.sup.-1 was not evident in the spectrum. Peaks unique to
caprolactam were not evident, suggesting that caprolactam was not
present in large amounts. It is uncertain whether the monomer
reacted with SiOH groups on the aerogel to form an ester linkage,
or whether ring opening and polymerization occurred at temperatures
well below that required in a bulk commercial process. A Raman
spectrum of the material was identical to that of caprolactam. It
is possible that the monomer remained unaltered, but interaction
between the caprolactam carbonyl and SiOH groups shifted the IR
absorbance peak slightly.
EXAMPLE 4
A sample of silica aerogel was exposed to vapors of caprolactam by
heating the sample in a lightly closed container with the monomer
at 150.degree. C. for 22 hours. The treated sample appeared smoky
and increased in mass by 6.1%, equating to 5.8% caprolactam in the
sample. The sample was then heated to 220.degree. C. for 30 minutes
to initiate polymerization of the monomer into nylon-6. The mass of
the sample dropped by .about.1.5% during heating due to
volatilization of some of the caprolactam. The final product was
smoky brown in color.
The modulus of this sample was 0.600 Kg/cm.sup.2, representing a
gain of approximately 55% over the untreated controls. An IR
spectrum of the material confirmed the presence of carbonyl-bearing
material in the product. As in Example 3, the spectrum was
complicated and did not cleanly match either the sample of pure
nylon-6, or the monomer. The interactions between the silica
aerogel and the organic material resulted in shifts of the energy
at which various bonds absorb, thus, shifting the spectrum relative
to pure compounds.
EXAMPLE 5
Aerogel substrate is placed in a sealed chamber. Phenol is
introduced into the chamber as a solid or a melt, kept separate
from the aerogel except for vapor access. The temperature is raised
to about 70.degree. C. and maintained until a substantially uniform
film of the phenol is deposited onto the aerogel substrate.
A catalytic amount of ammonia is added, followed by a slight
stoichiometric excess (1:1 to 1:1.3 mole ratio) of formaldehyde
added over about one hour in the form of a gas. (The formaldehyde
also can be added as an aqueous solution, preferably a 50% aqueous
solution, if kept physically separated from the aerogel except for
vapor access). The temperature of the system is held at about 65 to
70.degree. C. and at ambient pressure for an additional 2 to 4
hours. The temperature of the system is then raised to 130 to
200.degree. C. for about 2 hours while water vapor is allowed to
escape from the chamber. The chamber and coated aerogel are then
allowed to cool to ambient temperature under a nitrogen purge.
EXAMPLE 6
Trioxane is sublimed onto an aerogel substrate in a closed chamber
by heating solid trioxane to about 40 to 60.degree. C. while
avoiding direct contact between the solid trioxane and the aerogel.
Once a substantially uniform film of the trioxane is deposited onto
the surface of the aerogel, a small amount (from 0 to 5% of the
mass of the trioxane) of ethylene oxide is added to the chamber,
followed by the introduction of a catalytic amount of boron
trifluoride. The sealed reaction chamber is heated to 70 to
90.degree. C. for 2 to 3 hours, allowing the pressure to increase
autogenously. Following a purge of the chamber with nitrogen,
acetic anhydride is added and the temperature is maintained at 70
to 90.degree. C. for 1 hour with no purge. The chamber and coated
aerogel are then allowed to cool to ambient temperature under a
nitrogen purge.
Persons of ordinary skill in the art will recognize that many
modifications may be made to the foregoing without departing from
the spirit and scope thereof. The embodiment described herein is
meant to be illustrative only and should not be taken as limiting
the invention, which is defined in the following claims.
* * * * *